I have mentioned this "trick" before. Most digital cameras end up having a dilemma when trying to find the perfect exposure during the middle of the day. If you point at something like a tree, the exposure will be fine for everything on the ground and the trees but the sky will be washed out. The simplest way to add a nice blue sky back into your photo is using a simple gradient filter, specifically one that allows you control the position and rate of the gradient (sometimes called feathering). Here I am using the gradient filtering feature in picasa.